This generates more optimal codegen when using partial reductions with predication. ``` partial_reduce_*mla(acc, sel(p, mul(*ext(a), *ext(b)), splat(0)), splat(1)) -> partial_reduce_*mla(acc, sel(p, a, splat(0)), b) partial.reduce.*mla(acc, sel(p, *ext(op), splat(0)), splat(1)) -> partial.reduce.*mla(acc, sel(p, op, splat(0)), splat(trunc(1))) ```